Radioisotope processing



Aug. 5, 1969 Filed sept. 19, 196e C. P. RUIZ ETAL RADIOISOTOPE PROCESSING Aug. 5, 1969 c. P. Rulz ETAL RADIOISOTOPE PROCESSING Filed sept. 19. 196e 2 Sheets-Sheet 2 WILH United States Patent O 3,459,634 RADIOISOTGPE PROCESSING Carl P. Ruiz and Benjamin F. Rider, Fremont, and .lames M. Gerhart, Walnut Creek, Calif., assignors to General Electric Company, New York, N.Y., a corporation of New York Filed Sept. 19, 1966, Ser. No. 580,476 Int. Cl. GZlf 1/02; Ctllf 1/00, 15/00 U.S. Cl. 176-16 23 Claims ABSTRACT F THE DISCLOSURE A method of preparing and separating actinium-227 and thorium-228 from radium-226 is disclosed. Basically, this system consists of irradiating radium-226 with neutrons for a period suicient to form a mixture of actinium- 227, thorium-228 and radium-226, dissolving the mixture in an aqueous acid solution, extracting thorium-,228 with an organic solvent, adjusting the pH of the solution and extracting a-ctinium-227 with an organic solvent, drying the resulting organic solution, dissolving the dried material containing actinium-227 in an aqueous acid solution, adsorbing actinium-Z27 onto a column containing di-(2-ethylhexyl) phosphoric acid or a homolog thereof, then eluting the puriiied actinium-227 from the column.

The invention relates to the production and separation of the radioactive isotopes Ac-227 (actinium-227) and 'Th-228 (thorium-228) from Rat-226 (radium-226).

Existence of the isotopes Ac-227 and Thi-228 has been known for many years. However, to date these isotopes have remained the subject of laboratory investigations hampered by troublesome by-products.

This invention provides a practical and economical process for irradiating Ra-226 with neutrons to produce a mixture of Ra-226, Ac-227, and rl`h-228, which are separated n accordance with the process of this invention to provide industrial amounts of the isotopes Ac-227 and Th-228.

Radioactive isotopes are, at present, most commonly used in space power generators utilizing thermoelectric or thermoonic direct energy conversion or in dynamic systems utilizing Rankine or Brayton cycles. For the system to be satisfactory, the radioisotopes must have a halflie which is compatible with the mission life of the system. The radioisotope fuel must also be in a form capable of operating at high temperatures and for a power density high enough to produce the required heat with a reasonable size source. The radioactive radiation of the isotope must be low enough to prevent radiation damage or interference with other equipment in the system, and minimize exposure of the personnel associated with ground handling or in the spacecraft.

Ra-226 has a half-life of about 1620 years, which is far too long for most space power generator applications. Ac-Z27, on the other hand, has a half-life of about 21 years, and Th-228 has a half-life of about 2 years, which makes them ideally suited for a number of space power generator applications. Moreover, Ac-227 has a power density iive times greater than that of Cm-244 (curium- 244), and over one hundred times greater than Sr-90 (strontium-90), which at present are considered the most likely candidates for space power systems requiring relatively long half-life isotopes, i.e., greater than 5 years. 'Ih-228 has a slightly higher power density than Cm2\4-2, and has the advantage of a half-life which is over four times as long. Thus, from a half-life and power density standpoint, Ac-227 and Th-228 are well suited for space power application.

Briefly, the method of this invention includes the steps of irradiating Ra-226 with neutrons and to form Ra-227 which decays spontaneously with a half-life of 41 minutes to Ac-227. The irradiation of the Ra-226 is continued over a sufficient period so that a portion of the newlyformed Ac-227 is also irradiated with neutrons to form Ac-228 which decays with a half-life of about 6` hours to form T11-228.

Thereafter, the mixture of Ra-226, Ac-227, and Th- 228 is dissolved in an aqueous acid solvent, and separated by a combination of solvent extraction and chromatographic column separations.

The Ac-227 and Th-ZZS are recovered separately to provide the radioisotope heat and neutron sources required for space power generator applications, and the lla-226 is recovered and re-irradiated to produce additional Ac-227 and T11-228.

The invention will be more fully understood from the following detailed -description and the accompanying drawings, in which:

FIG. l is a flow sheet showing the presently-preferred process of the invention giving high purity products; and

FIG. \2 is a ilow sheet showing an alternate process of the invention.

Referring to FIG. l, a sample of Ra-226 in the form of RaBr2 (radium bromide) was irradiated with neutrons in a nuclear reactor 10. The RaBrz was encapsulated in 1A diameter quartz vials and had a surface-to-mass configuration greater than 1.0 cm2/gm. to limit the amount of resonance self-shielding. If the surface-to-mass ratio conguration is lower than about 1.0 cm.2/ gm., the effective cross section of the Ra-226 for capturing neutrons decreases because of resonance self-shielding.

A relatively high neutron iiux was used in irradiating the Ra-226 to minimize the decay losses of Th-228 during irradiation. For example, fluxes of about 1014 neutrons/cm.2/sec. were used to obtain satisfactory results. Irradiation of Ra-226 with neutrons produces the following reactions:

From the above, it can be seen that the Th-228 also captures neutrons to produce Th-229 which is diluent and, therefore, limits the exposure of the Ra-226 to neutrons to avoid excessive conversion of the useful Th-228 to the undesired Th-229.

After irradiation, the quartz capsules with a mixture of lla-226, Ac-227, and Th-228 were placed in a capsule breakage chamber 12 which contained 2 molar HNO3 (nitric acid). The chamber was sealed, and the capsules broken remotely so that the Ra, Th, and Ac dissolved in the aqueous nitric acid. A relatively inert gas such as helium, nitrogen, or air was bubbled into the nitric acid solution to sweep radon gas produced by the natural decay of Ra-226 from the chamber and into a charcoal trap 14 cooled with liquid nitrogen, although adequate adsorption on the charcoal trap was also achieved at room temperature operation. A vacuum pump 15 connected to the charcoal trap kept the breakage chamber at subatmospheric pressure, conveniently about 3A atmospheric pressure.

The aqueous acid solution of Ra, Th, and Ac was vigorously mixed in a irst mixer or extractor 16 with 0.25 molar TTA (thenoyltritluoroacetone) dissolved in toluene at pH 2. Other solvents such as the homologues of toluene or their equivalents can be used. Homologues of TTA can also be used. TTA is a chelate which combines with the Ra, Ac, and Th to form compounds soluble in organic solvents. The pH of the TT A in the first mixer 16 is adjusted to about 2 by the addition of sodium hydroxide. At a pH greater than about 1, Th is extractable into TTA, whereas the Ac is extractable at a pH greater than about 4. With an organic-to-aqueous volume ratio of 2, 80% of the Th was extracted into the organic phase in about five minutes at pH 2 with vigorous mixing of the aqueous and organic phases. The yield approached 94% for an extraction time of thirty-tive minutes, and there was no extraction of Ac.

Homologues of TTA have the basic formula R-o-oHT-o-R' in which:

(1) Thenoyltriuoroacetone (TTA) has:

R=-C CH R'=CF3 H U Ho CH (2) Acetylacetone has:

R=CH3 R'=-CH3 (3) Dibenzoylmethane has:

(5) Dichlorophenylheptauorohexanedione has three isomers:

The organic phase with the Th extracted in the first mixer 16 was placed in a first back-extractor 18 and mixed vigorously with 8 molar HNOS to back-extract the Th into an aqueous phase. The back-extraction can be performed with acidities greater than about 1 M.

The remaining aqueous phase in the mixer 16 was transferred to a second mixer or extractor 20 and adjusted to pH 6 by the addition of ammonium hydroxide and mixed vigorously with .25 molar TTA in toluene to 4 extract the Ac. The acidity of the solution gradually increased during the extraction according to the following equation:

Therefore, the pH of the extraction system for the Ac was continually adjusted during the initial few minutes of the extraction by the addition of NH4OH (ammonium hydroxide), during which time most of the Ac was extracted.

The organic phase with the Ac was transferred from the second extractor 20 to a second back-extractor 22 and mixed vigorously with 8 molar HNO3 which backextracted the Ac into the aqueous phase. This backextraction can be performed with acidities greater than about 0.1v M. Any residual Th is back-extracted with the Ac at this stage.

The aqueous phase in the second mixer contained the Ra-226 and was transferred to a storage tank 24 for further processing and recovery, as described in detail below.

The aqueous thorium solution in the rst back-extractor was passed through a Dowex 1 x 8, 200-400 mesh anion exchange chromatographic column 26, 3 mm. in diameter by 5 cm. long and preconditioned with 8 M HNO3. This column will work with other anion exchange resins of the quaternary amine type having cross linking other than 8%, e.g., 4%, and other mesh sizes, eg., 100-200 mesh, and can be operated at elevated temperatures up to 70 C. for improved adsorption rates. The Th was adsorbed in such a column at concentrations of HNO3 greater than about 5 molar. Ac and Ra were not adsorbed at any HNO3 concentration. After passing the Th extract through the first chromatographic column, it was washed with 8 column volumes of 8 molar HNO3. 1 column volume is dened as the volume of liquid occupying the void space in 1 bed volume. The Th was then eluted from the rst chromatographic column with 4 column volumes of 0.5 molar HNO3 and collected in an evaporator 28 where the volume of the Th solution is reduced to a convenient size. Thereafter, the Th solution is placed in a storage tank 29 for further use, when desired.

The aqueous phase of the second back-extractor with the dissolved Ac and some residual Th and traces of Ra is evaporated to dryness in a second evaporator 30 and dissolved in a small volume of .05 molar HCl (hydrochloric acid) and passed through a reverse phase chromatographic column 32 which is then eluted with 0.10 molar HCl at a ilow rate not exceeding 2 column volumes per minute. The rst 3 column volumes eluted from the reverse phase chromatograph contain Th and Ac daughter products as well as some residual lla-226. The Ac-227 fraction is collected in column volumes 4 through 6 and placed in a third evaporator 34.

The reverse phase chromatographic column used 20% HDEHP (di(2ethylhexyl) phosphoric acid) supported on Chromosorb operated at 87 C. This column can be operated at lower temperatures at the expense of reduction in rate of adsorption and desorption. Ra and Ac were adsorbed at HC1 concentrations less than .001 molar and 0.05 molar, respectively. The Th was adsorbed strongly at all HCl concentrati-ons. Excellent separation was achieved by eluting the Ra at .005 molar and the Ac at 0.1 molar HCl. Th was subsequently eluted with a mixture of 6 molar HCl plus l molar HF (hydrouoric acid).

Other homologues of HDEHP can be used in place of HDIEHP. Homologues of HDEHP have the basic formu a:

in which:

hich:

(1) Mono-Z-ethylhexylphosphoric acid has:

(2) Dinbuty1phosphoric acid has:

20 (3) Mouododecylphosphoric acid has:

(3) Bis(3,5,S-trimethylheXyDphosphorc acid has:

IICI

(4) Bis(diisobutylmethyDphosphoI-ic acid has:

(6) Mono(poctylphenyl) phosphoric acid has:

acid) having the basic formula:

The Ac can be stored in a tank 36 until further use, such as a neutron source when mixed with Be (beryllium). This useful combination is obtained by treating the Ac solution in storage tank 36 with a mixture of HNO3 and HF at a molarity of greater than 0.1. The product is ACFS (actinium fluoride) which is easily converted to elemental Ac for best contact with Be in accordance with the following equation:

heat SBeFz In the iinal separation of Ac-227 to yield a high-purity product, i.e., the chromatographic separation from the reverse phase HDEHP column, the A'c-227 is probably combined with the HDEHP in the eluted fraction. This results in the presence of a considerable amount of HDEHP in the final Ac-227 product which thus requires additional purification.

The additional purification is performed by the precipitation of the Ac-227 as the compound AcF3 from the mixture of HNOS and HF. For some applications, the additional purification is not performed, such as when an actinium glass as desired. Instead of treating the eluate with the mixture of HNO3 and HF, the eluate from the reverse phase HDEHP column is evaporated to produce a residue which has the properties of a glass due to the presence of the phosphate resulting from the decomposition of the HDEHP. The properties of the glass can be modified or adjusted to desired values by treating the eluate from the HDEHP column with additives such as sodium, iron, etc. to obtain a glass having a low enough melting point to obtain a melt, and low leachability.

The Ra in the aqueous phase in the storage tank 24 is precipitated as radium sulfate in a reaction tank 38 and converted to radium carbonate in an autoclave 40 by heating with sodium carbonate solution. The radium carbonate is then dissolved in HBr (hydrobrornic acid) in a convertor to form radium bromide which is then encapsulated at 44 and ready for irradiation.

The chemical processing procedure can be used as described above, or it can be varied to meet certain requirements. For example, if high-purity material is not required, the separation process can be stopped after the separate TTA solvent extractions of the Th and Ac from the aqueous nitric acid.

FIG. 2 shows an alternate procedure for preparing Ac-227 and Th-228 in accordance with this invention.

A radium bromide capsule is irradiated with neutrons, broken in a sealed chamber 50 and dissolved in nitric acid as previously described. During the breakage and dissolution, helium gas is -bubbled through the solution to sweep radon gas into a cooled charcoal trap like that referred to with respect to FIG. l.

After about `1/2 minute helium purge, the solution is adjusted to ph 2 with ammonium hydroxide. 2 volumes of .25 molar TTA in toluene are added to the aqueous phase in a rst extractor 52, and the mixture stirred vigorously for 5 minutes to extract thorium. The TTA phase is separated from the aqueous phase and placed in a first back-extractor 54. The 5-minute extraction is repeated with another double volume of TTA and the two organic phases are combined in the first back-extractor 54. The thorium is back-extracted in the aqueous phase in 8 molar HNO3.

The aqueous phase from the first extractor with dissolved actinium is treated in a second extractor 56 with .25 molar TTA in toluene with the aqueous phase pH adjusted to 6 with ammonium hydroxide to extract the actinium. Two S-minute extractions are performed as in the rst extractor 52 with careful control of the pH by addition of dilute ammonium hydroxide. The two TTA fractions are combined in a second back-extractor 58, and the actinium is back-extracted into an aqueous phase with 8 molar HNO3.

The Ac solution from the second back-extractor is passed through a chromatographic column 60 of the conventional type known as a Dowex 1 x 8, ZOO-400 mesh, 3 mm. by 5 cm. column, preconditioned with 8 molar HNO3. The Th solution from the first back-extractor is passed through the column after the Ac solution.

The column is washed with 4 more column volumes of 8 molar nitric acid, and the wash solution discarded. The Th-228 is eluted with 4 column volumes of .5 molar HNO3 and placed in an evaporator 62 where it is evaporated to a small volume and then transferred to a storage tank 64. The Ac solution which passes through the chromatographic column 60 is placed in a second evaporator 66, evaporated to dryness, and then dissolved in a small volume of .05 molar HCl, which is then loaded on a preconditioned HDEHP column 68 prepared as described above with reference to FIG. l, and eluted with 0.10 molar HCl. Column volumes 1 through 3 contain impurities, and are discarded. Arr-227 is collected in column volumes 4 through `6 in an evaporator 70 and then treated as previously described with respect to FIG. l and placed in storage tank 72.

From the foregoing description, it will be apparent that the Th and Ac can also be extracted simultaneously with TTA at pH 6, and thereafter separated in accordance with the column chemistry described with respect to FIG. 2.

We claim:

1. The method of preparing actinium-227 and thorium- 228 from radium-226 which comprises the steps of:

(a) irradiating radium-226 with neutrons to form actinium-227;

(b) continuing said neutron irradiation so that some of said actinium-227 forms actinium-228, which decays into thorium-228, whereby a mixture of radium- 226, actinium-227 and thorium-228 is produced;

(c) performing a single cycle comprising the steps of:

(l) dissolving said mixture in an aqueous acid solvent;

(2) adjusting the pH of said aqueous acid solution to about 2;

(3) extracting thorium-228 with an organic solvent;

(4) adjusting the pH of said aqueous acid solution to about 6;

(5) extracting actinium-227 with an organic solvent;

(d) after said single cycle evaporating the actinium- 227 solution to dryness;

(e) dissolving the dried actinium-227 material in an about 0.05 molar aqueous acid solution;

(f) passing the resulting solution through a column containing a composition selected from the group consisting of di-(2-ethylhexyl) phosphoric acid and its homologs; and

(g) eluting actinium-227 from said column with an about 0.1 molar aqueous acid solution, whereby said actinium-227 is substantially free from thorium-228, radium-226 and impurities.

2. The method according to claim 1 wherein said organic solvent used for said extraction of thorium-228 comprises a compound selected from the group consisting of thenoyltrifluoroacetone and homologs thereof.

3. The method according to claim 2 wherein said organic solvent comprises about 0.25 molar thenoyltrifluoroacetone dissolved in another organic solvent at a pH of about 2.

4. The method according to claim 1 wherein said organic solvent used for said extraction of actinium-227 comprises a compound selected from the group consisting of thenoyltriiiuoroacetone and homologs thereof.

5. The method according to claim 4 wherein said organic solvent comprises about 0.25 molar thenoyltrifluoroacetone dissolved in another organic solvent at a pH of about 2.

6. The method according to claim 1 wherein said elution of actinium-227 from said column is performed with about 0.1 molar hydrochloric acid.

7. The method according to claim 6 wherein said elution includes washing the actinium-227 from said column with about six column volumes of 0.1 molar hydrochloric acid and collecting volumes 4 through 6 separately from volumes 1 through 3.

8. The method according to claim 6 which includes treating the eluted actinium-227 with a solution comprising nitric acid and hydrofluoric acid at an acidity greater than about 0.1 molar to form actinium fluoride.

9. The method of claim 8 further including the steps of mixing said actinium uoride with beryllium, and heating the mixture to drive off beryllium fluoride as a gas, whereby an alloy of actinium and beryllium is formed.

10. The method according to claim 6 wherein subsequent to the elution of actinium-227 from said column, thorium-228 is eluted with a mixture comprising about 6 molar hydrochloric acid and about 1 molar hydroluoric acid.

11. The method according to claim 1 further including the step of eluting radium-226 from said column with an about 0.005 molar hydrochloric acid solution.

12. The method according to claim 1 wherein the organic solution containing actinium-227 is contacted with an about 8 molar aqueous acid solution to back-extract the actinium-227 into the aqueous phase before said step of evaporating to dryness.

13. The method according to claim 1 wherein the organic solution containing thorium-228 is back-extracted with an about 8 molar aqueous acid solution, to return the thorium-228 to the aqueous phase, then the aqueous solution is concentrated by evaporation.

14. The method according to claim 1 which includes recovering the residual Ra-226 and re-irradiating it with neutrons to form additional Ac-227 and Th-228.

15. The method according to claim 14 which includes 19. The method according to claim 18 which includes the step of passing the relatively inert gas and radon gas through a filter.

20. The method according to claim 19 in which the filter includes charcoal, and including the step of cooling the charcoal below about C.

21. The method according to claim 1 which includes the step of dissolving the mixture of Ra-226, Ac-227, and Th-228 in an aqueous acid solvent in a container at less than atmospheric pressure to prevent the out leakage of radon gas given off by the mixture.

22. The method according to claim 1 in which the surface-to-mass ratio of the Ra-226 is greater than about 1.0 cm.2/ gm.

23. The method according to claim 1 which includes the step of adding a base during the extraction of the Ac- 227 to keep the pH of the extraction solution at least 6.

References Cited UNITED STATES PATENTS 2,632,763 3/1953 Hagemann 23-339 2,723,901 11/1955 Hagemann et al. 23-23 2,856,418 10/1958 Calvin 23-339 2,916,349 12/1959 Crandall et al. 23-339 OTHER REFERENCES MLM-1204, September 1964, p. 19.

MLM-558-Del-l, March 1960, pp. 4, 6-9, 15, 16, 19 21, 33, 34, 43.

MLM-773-Rev., March 1960, pp. 3-6.

MLM-1297, February 1966, PP. 3-11.

MLM-1176, September 1963, pp. 3, l1.

ABCD-3084, p. 25, March 1951.

Nuclear Engineering Handbook, November 1958, sec. 11, pp. 91-97, sec. 14, p. 31.

MLM-622-Del-1, 1959, p. 9.

I. Amer. Chem. Soc., vol. 78, November 1956, pp. 59535954 by Danon.

I. Amer. Chem. Soc., February 1950, vol. 72, pp. 768- 774 by French Hagemann.

I. of Chemical Education, vol. 36, No. 9, September 1959, pp. 462-465 by Choppin.

Can. I. Chem., vol. 37, 1959, pp. 1094-1103, by Cabell.

CARL D. QUARFORTH, Primary Examiner H. E. BEHREND, Assistant Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 3 ,459 ,634 August S 1969 Carl P. Ruiz et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby Corrected as shown below:

Column l, line Z9, "The", first occurrence, should read This Column Z, line 2, cancel "and"; line 55, "(18-day should read [18 days) Column 5, line Z, "Di(2ethyl hexylphosphorc" should read Di(2-ethylhexyl)phosphorc Column 7, line l0, "AcBelS" should read ZAcBel3 line Z "as" should read 1s line 56, "ph" should read pH Signed and sealed this 7th day of July 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Commissioner of Patents Edward M. Fletcher, J r.

Attesting Officer 

